Novel additive comprising lead and/or a lead alloy intended to treat baths of liquid steel

ABSTRACT

The present invention relates to an additive in the form of flux-cored wire for treating baths of liquid steel with a view to obtaining steels having a high lead content. The additive comprising metallic lead and/or one or more lead alloys according to the invention for treating baths of liquid steel, and is in the form of flex-cored wire composed of a metal sheath and a finely divided filling material, the latter being composed of a powder of metallic lead and/or of lead alloy and of a powder containing a material capable of releasing a gas, which is inert with respect to the liquid steel, at the temperature of the liquid steel bath. Characteristically, the powder of metallic lead and/or of lead alloy includes a particle size fraction G R  between 200 μm and 500 μm and the particle size fraction G R  has the following characteristics: —through a 200 μm sieve: G R &lt;5%; —through a 300 &amp; μm sieve: 90%  3 G R   3  10%; —through a 400 μm sieve: 40% £ G R &lt;100%; —through a 500 μm sieve: 100%  3 G R   3  90%.

The present invention relates to an additive in the form of flux-cored wire for treating baths of liquid steel in order to obtain steels having a high lead content.

Lead is well-known for improving the machinability of steels since, as it is insoluble in steel, it forms exogenous lead inclusions (nodules) which act as a lubricant and promote the severance of turnings during machining of rolled steels. However, the use of lead does have some serious drawbacks owing to its toxicity, its high density (greater than that of liquid steel) and its low melting point. It is introduced into baths of liquid steel by injecting beads or shots through an immersed nozzle or even in the form of flux-cored wire, this latter technique generally being recognised as more adaptable and more reliable.

Cumulative yields currently obtained using conventional flux-cored wires do not allow large amounts of lead to be added, because a considerable amount of toxic fumes would be produced, posing considerable drawbacks with regard to staff hygiene and safety.

Document EP 0 316 921 discloses a lead-containing additive for steel baths, in the form of flux-cored wire composed of a metal sheath and a finely-divided filling material, said material containing metallic lead and/or lead alloys as well as a material containing lime and releasing carbon dioxide (CO₂) at the temperature of the bath of liquid steel. The release of CO₂ into the steel bath creates strong turbulence around the flux-cored wire which causes the lead particles to blend into the liquid steel and encourages their movement within the bath, thus improving their distribution in the bath of liquid steel. Furthermore, the use of this additive has made it possible to limit the amount of toxic fumes emitted and to better control the addition process whilst increasing the cumulative yield thereof compared with those yields observed previously.

However, it has been found that the use of this type of additive does not make it possible to achieve a uniform distribution, within the bath of liquid steel, of lead inclusions throughout the melt. In addition, the end steel product obtained using this additive does not have a homogeneous distribution of lead inclusions. Furthermore, the cumulative yield of lead in the bath of liquid steel still remains below 70%.

The object of the present invention is to overcome these drawbacks by proposing a novel additive comprising a metallic lead powder and/or lead alloy powder having a very specific size and particle size distribution, said powder being combined with a compound able to provide homogeneous distribution of the lead in the bath of liquid steel.

To this end and in accordance with a first aspect, the invention relates to an additive comprising metallic lead and/or lead alloys, for treating baths of liquid steel, said additive being in the form of flux-cored wire composed of a metal sheath and a finely-divided filling material, the latter being formed of a metallic lead powder and/or lead alloy powder and of a powder of a compound able to release a gas, which is inert with respect to the liquid steel, at the temperature of the bath of liquid steel, said additive being characterised in that said metallic lead powder and/or lead alloy powder consists of a particle size fraction G_(R) between 200 μm and 500 μm, and in that said particle size fraction G_(R) has the following characteristics:

-   -   through a 200 μm sieve: G_(R)≦5%;     -   through a 300 μm sieve: 90%≧G_(R)≧10%;     -   through a 400 μm sieve: 40%≦G_(R)≦100%;     -   through a 500 μm sieve: 100%≧G_(R)≧90%.

In accordance with a second aspect, the invention relates to a method for treating baths of liquid steel using an additive comprising metallic lead and/or one or more lead alloys, the method comprising a step of adding to said steel baths an additive in the form of a flux-cored wire composed of a metal sheath and a finely-divided filling material, the latter being formed of a metallic lead powder and/or lead alloy powder and of a powder of a compound able to release a gas, which is inert with respect to the liquid steel, at the temperature of the bath of liquid steel, said metallic lead powder and/or lead alloy powder consisting of a particle size fraction G_(R) between 200 μm and 500 μm and having the following characteristics:

-   -   through a 200 μm sieve: G_(R)≦5%;     -   through a 300 μm sieve: 90%≧G_(R)≧10%;     -   through a 400 μm sieve: 40%≦G_(R)≦100%;     -   through a 500 μm sieve: 100%≧G_(R)≧90%.

In accordance with a third aspect, the invention relates to the use of the additive comprising metallic lead and/or one or more lead alloys described above, for treating baths of liquid steel.

In accordance with a fourth aspect, the invention also relates to any rolled steel product having a high lead content, obtained by the aforementioned method and characterised in that the lead nodules are smaller than 100 μm and a large majority thereof, equal to 80%, are distributed randomly within the rolled steel, as shown in FIG. 3. This distribution affords the rolled steel optimum machinability characteristics.

The use of this novel additive makes it possible to improve, in a very significant manner, the cumulative yields of lead and therefore also provides the possibility of adding greater amounts under conditions which are satisfactory with regard to hygiene and safety. It also makes it possible to achieve a better final distribution of the lead nodules within the solid steel whilst reducing the occurrence of remanence and contamination of the refractory materials of the baths used to treat these steels. The cost of producing these steels is thus improved.

Other features and advantages of the invention will become clear upon reading the following detailed description and embodiments, as well as by viewing the accompanying figures, in which:

FIG. 1 shows the characteristics of the sizes and particle size distributions of the particle size fraction G_(R);

FIG. 2 shows the variation in the yield of lead as a function of the amount of additive added to the steel bath expressed as length of the flux-cored wire added per tonne of liquid steel;

FIG. 3 shows schematic views of different types of distribution of lead nodules within the end product of solid steel.

FIG. 4 is a diagram allowing the calculation of the minimum distance to the closest lead nodule.

The present invention relates to a novel additive comprising metallic lead and/or one or more lead alloys, for treating baths of liquid steel in order to obtain steels having a high lead content. As is known, this additive is in the form of flux-cored wire composed of a metal sheath and a finely-divided filling material, the latter being formed of a metallic lead powder and/or lead alloy powder and of a powder of a compound able to release a gas, which is inert with respect to the liquid steel, at the temperature of the bath of liquid steel.

Advantageously, said metallic lead powder and/or lead alloy powder consists of a particle size fraction G_(R) between 200 μm and 500 μm. This particle size fraction G_(R) is preferably in the form of small granules or very fine beads.

In a distinctive manner, said particle size fraction G_(R) has the following characteristics:

-   -   through a 200 μm sieve: G_(R)≦5%;     -   through a 300 μm sieve: 90%≧G_(R)≧10%;     -   through a 400 μm sieve: 40%≦G_(R)≦100%;     -   through a 500 μm sieve: 100%≧G_(R)≧90%.

These particle size characteristics are shown schematically in the accompanying FIG. 1.

This particle size distribution (contained within the region shown in FIG. 1) affords the flux-cored wire optimum filling properties, resulting in effective metallurgical treatment of baths of liquid steel. The choice of a particle size distribution of this type ensures a level of residual porosity which is a lot lower than that observed with flux-cored wires produced from a conventional lead powder. Porosity is thus between 5% and 20% maximum, whereas this value is generally between 15 and 40% in the case of a conventional wire.

The metal sheath surrounding the additive is formed of a material able to dissolve in the steel bath at a speed which is sufficiently quick to allow said additive to be released, without introducing undesired components into the bath. Preferably, the metal sheath is made of unalloyed mild steel. It is between 0.1 and 1 mm thick, preferably between 0.2 and 0.6 mm thick.

Furthermore, the diameter of the flux-cored wire according to the invention is between 5 and 20 mm, preferably between 9 and 15 mm.

The additive according to the invention is in the form of flux-cored wire containing 100 to 1000 g of lead per metre of wire.

As for the powder of a compound which is able to spontaneously release a gas, which is inert with respect to the liquid steel, at the temperature of the bath of liquid steel (approximately between 1550 and 1650° C.), this is also in finely divided form and has a particle size less than 1 mm, preferably less than 0.5 mm.

Advantageously, the release of gas bubbles in the bath of liquid steel creates an upward current which leads to a highly random distribution of the lead inclusions formed from the particle size fraction according to the invention, thus resulting in a uniform distribution of said inclusions within the bath of liquid steel.

In a particular embodiment, the compound able to spontaneously release a gas, which is inert with respect to the liquid steel, is a mineral compound, such as limestone (calcium carbonate) or non-calcined dolomite, and said gas which is inert with respect to the liquid steel is carbon dioxide. In this case, the mineral compound is used in an amount of 3 to 30% by weight based on the weight of the metallic lead and/or the lead alloy or alloys used.

In accordance with a second aspect, the invention relates to a method for treating baths of liquid steel using an additive comprising metallic lead and/or one or more lead alloys, the method comprising a step of adding to said bath an additive in the form of the flux-cored wire described above.

A flux-cored wire containing a powder of this type makes it possible to obtain a yield of lead within the bath of liquid steel which is greater than that obtained using a conventional wire or even using the wire disclosed in document EP 0 316 921.

FIG. 2 shows industrial results regarding the yield of lead as a function of the amounts added (the variation in the yield of lead as a function of the amount of additive added to the bath of steel being expressed as length of flux-cored wire per tonne of steel).

The yield of lead is defined by the following equation:

Y _(Pb)=(C _(F) −C _(I))/C _(A)

In this equation:

-   -   C_(I) is the initial lead content in the bath of liquid steel;     -   C_(F) is the final lead content obtained in the bath of liquid         steel;     -   C_(A) is the desired lead content in the bath of liquid steel;     -   Y_(Pb) is the cumulative yield of lead.

The flux-cored wire containing the lead powder and/or a lead alloy of which the particle size conforms to the requirements described within this invention makes it possible to obtain a yield of lead which is greater than that obtained using a conventional powder. It also makes it possible to obtain yields which are very even and consistent irrespective of the length of wire injected into the bath of liquid steel. The more greatly reduced dispersion thus makes it possible to increase, very significantly, the likelihood of obtaining the desired amount of lead in the end steel.

Owing to a greater yield, the conditions with regard to hygiene and safety during treatment of baths of liquid steel are also considerably improved. Fewer toxic fumes are released above the bath. The occurrence of lead sedimentation at the bottom of the bath, and of contamination of the refractory walls of baths is also greatly reduced.

The additive according to the invention is added to the bath of liquid steel before casting. Depending on the final desired lead content, an amount ranging from 0.1 to 10 kg of additive in the form of flux-cored wire is added per tonne of liquid steel to be treated. The flux-cored wire is unwound into the bath of steel at a speed ranging from 50 to 200 m/min, preferably from 100 to 150 m/min.

The two following examples demonstrate the increased values obtained for the yield of lead by using the novel additive according to the invention.

EXAMPLE 1

Flux-cored wire with an outer diameter of 13.6 mm

Strip between 0.35 and 0.40 mm thick

Amount of calcium carbonate in the mixture: 6.3% by weight

Metric weight of the wire: 970 g/m

Injection rate: 120 m/min

Weight of liquid steel in the bath: 95 t

Desired lead content: 0.260%

Lead content obtained after treatment: 0.248%

Cumulative yield of lead obtained in the flux-cored wire is: 71.8%

EXAMPLE 2

Flux-cored wire with an outer diameter of 13.6 mm

Strip between 0.35 and 0.40 mm thick

Amount of calcium carbonate in the mixture: 5.8% by weight

Length of wire injected: 334 m

Injection rate of the wire: 150 m/min

Weight of liquid steel in the bath: 115 t

Desired lead content: 0.200%

Initial lead content in the bath of liquid steel: 0.009%

Lead content obtained after treatment: 0.191%

Cumulative yield of lead obtained in the flux-cored wire is: 72.0%

Since the particle size of the lead particles contained in the flux-cored wire was selected within a very small range of 200 to 500 μm, the lead inclusions, which are insoluble in liquid steel, are distributed uniformly throughout the bath.

Advantageously, the reduced size of the lead inclusions makes it possible to significantly reduce their sedimentation at the bottom of the bath. This leads to an even lead content in the bath of liquid steel from the onset of draining of the bath until the end of the casting process. The solidified product thus has a more homogeneous lead content, irrespective of the amount of liquid steel yet to be cast. This is demonstrated in the following example.

EXAMPLE 3

Flux-cored wire with an outer diameter of 13.6 mm

Strip between 0.35 and 0.40 mm thick

Amount of calcium carbonate in the mixture: 6.5% by weight

Amount of powder injected by flux-cored wire: 297 kg

Injection rate of the wire: 120 m/min

Bath weight: 95 t

Desired lead content: 0.260%

Lead content obtained in the first blooms cast: 0.252%

Lead content obtained in blooms in the middle of the bath: 0.245%

Lead content obtained in blooms once the bath has been drained: 0.249%.

The term ‘bloom’ means a unit of solidified steel (steel ingot having an annular, rectangular or polygonal cross-section).

Furthermore, the use of the flux-cored wire containing a finely-divided lead powder in accordance with the invention makes it possible to reduce washings of the baths used to produce liquid steels having a high lead content. The refractory materials of the bath are less contaminated by considerable lead infiltration. The steelmaker will observe less lead residue in the systems used to open/close the tap hole as well as in the joins between the fireclay bricks.

The amount of waste rolled products (bars) made of steel having a high lead content is greatly reduced owing to the use of the additive containing a lead powder and/or lead alloy powder of which the particle size is disclosed within the present invention. The bars are rejected if the size and distribution of the lead nodules does not correspond to the specification stipulated by the steelmaker's client. Owing to the additive according to the invention, 100% of bars conform to these specifications, whereas the use of a flux-cored wire containing a conventional lead powder may lead to up to 30% wastage.

As well as being smaller, the lead nodules are distributed more effectively within the rolled product, thus promoting the properties of machinability. There are currently no standard or international methods for characterising the distribution of lead nodules in laminated products, which is why we have specifically developed a criterion for qualifying the distribution of the population of lead products in laminated products.

For this reason, the applicant has specifically developed a criterion for qualifying the distribution of the population of lead nodules in laminated products. The applicant has therefore defined distribution indices and the associated criteria as well as the conditions for experimental measurements.

Owing to a comprehensive study using digital simulation modelling the different distributions shown in FIG. 3, the applicant has revealed relevant indices making it possible to qualify these distributions. The applicant also determined the thresholds associated with each of these indices. In this way, it has been proven that a distribution index I_(R) greater that 1.4% makes it possible to differentiate, with a tolerance interval of 99%, between a random distribution and other types of distribution.

The index I_(R) is defined as follows:

$I_{R} = {\left( {100/D} \right) \star \left( {\sum\limits_{i = 1}^{NI}{\left( d_{i} \right)/{NI}}} \right)}$

where:

-   -   I_(R): distribution index     -   D: diagonal of the analysis zone     -   D_(i): minimum distance between the lead nodules (the closest         nodules—FIG. 4)     -   NI: number of lead nodules with an associated minimum distance,

The lead nodules are randomly distributed (thus promoting machinability) when I_(R) is greater than 1.4%.

This index I_(R) is only relevant if there is an increased number of lead nodules. This number was fixed at 500. A method of specific analysis was thus developed.

The lead nodules were characterised on the surface of a sample taken from the middle of a laminated steel bar having a diameter greater than 40 mm, and viewed in the direction of lamination. The surface of the sample taken is polished until it has a sheet of 1 μm.

The lead nodules are identified and characterised by observing the surface of the sample using a scanning electron microscope equipped with a backscattered electron detector (SEM-BSE) connected to an image analyser. Using this method of observation and owing to the chemical contrast, the lead nodules are shown with a shade of grey which is considerably darker than that of the steel matrix and of other types of inclusion (such as sulphides, oxides, nitrides, etc.), which means they can be easily distinguished and isolated.

The measurement method involves observing a surface of at least 25 mm² within a square region centred in the middle of the bar. All the lead nodules having a small Feret diameter greater than 2 microns are included and measured. More than 500 lead nodules are to be included. For each of these nodules, the positional parameters (X and Y coordinates within the point of reference of the region examined) and the main morphological parameters (nodule surface, Feret diameter, etc.) are recorded in a results file.

The distribution parameters are then calculated in order to reveal the distribution of the lead nodules, which makes it possible to optimise the machinability properties of the product. With reference to the accompanying FIG. 3, many types of distribution are taken into consideration: random (FIG. 3 a), clustered (FIG. 3 b), in strips (FIG. 3 c) or grid-like (FIG. 3 d). In order to obtain a machinability aptitude which is sufficiently advanced from that known for laminated steels having a high lead content and treated using conventional flux-cored wires, it has been established that the amount of nodules distributed randomly has to be as great as possible and is preferably equal to at least 80%.

The use of an additive in the form of flux-cored wire containing a lead powder, of which the particle size is as defined in FIG. 1, makes it possible to obtain a product made of homogeneous rolled steel containing lead nodules which are very small, smaller than 100 μm, and are distributed in a largely random manner. This distribution affords the steel improved machinability properties compared to those obtained with steel treated using conventional flux-cored wires or those disclosed in document EP 0 316 921.

The use of the novel additive, comprising metallic lead and/or a lead alloy, according to the invention for treating baths of liquid steel in order to obtain steels having a high lead content has numerous advantages and generally leads to:

-   -   an improvement in the cumulative yield of lead in the bath of         liquid steel;

an improvement in the conditions for producing and treating steels having a high lead content. More specifically, the increase in the cumulative yield makes it possible to reduce the emission of toxic fumes and thus results in improved conditions, with regard to hygiene and safety, for steelworkers;

-   -   a significant improvement in the distribution and fineness of         the lead inclusions in the liquid steel which makes it possible         to keep the lead content of the steel even, from the onset of         casting until the bath has been completely drained;     -   a significant reduction in the lead sedimentation at the bottom         of the bath, and thus near-disappearance of the lead residues in         the systems used to open/close the tap holes as well as in the         joins between the fireclay bricks;     -   an improvement in the final machinability properties of rolled         steel products owing to a better distribution of lead nodules in         the rolled steel, this distribution being largely random and         characterised by an innovative method developed specifically in         order to compensate for the lack of an international standard         method;     -   a significant reduction in the amount of rolled products         rejected owing to internal defects caused by the presence of         large, undesirable lead nodules and/or lead nodules which are         distributed ineffectively within the rolled steel products. 

1. Additive comprising metallic lead and/or one or more lead alloys, for treating baths of liquid steel, said additive being in the form of flux-cored wire composed of a metal sheath and a finely-divided filling material, the latter being formed of a metallic lead powder and/or lead alloy powder and of a powder containing a material able to release a gas, which is inert with respect to the liquid steel, at the temperature of the bath of liquid steel, said additive being characterised in that said metallic lead powder and/or lead alloy powder consists of a particle size fraction G_(R) between 200 μm and 500 μm, and in that said particle size fraction G_(R) has the following characteristics: through a 200 μm sieve: G_(R)≦5%; through a 300 μm sieve: 90%≧G_(R)≧10%; through a 400 μm sieve: 40%≦G_(R)≦100%; through a 500 μm sieve: 100%≧G_(R)≧90%.
 2. Additive according to claim 1, wherein the metal sheath is made of unalloyed mild steel.
 3. Additive according to claim 1, wherein the metal sheath is from 0.1 to 1 mm thick, preferably from 0.2 to 0.5 mm thick.
 4. Additive according to claim 1, wherein the flux-cored wire has a diameter from 5 to 20 mm, preferably from 9 to 15 mm.
 5. Additive according to claim 1, wherein the filling material has a particle size which does not exceed 1 mm.
 6. Additive according to claim 1, wherein the flux-cored wire contains from 100 to 1000 g of lead per metre.
 7. Additive according to claim 1, wherein the material able to release a gas, which is inert with respect to the liquid steel, is a mineral compound formed of limestone (calcium carbonate) or non-calcined dolomite, the gas released thus being carbon dioxide.
 8. Additive according to claim 7, wherein the mineral material is present in an amount of 3 to 30% by weight based on the weight of lead or lead alloy (or alloys) used.
 9. Method for treating baths of liquid steel using an additive comprising metallic lead and/or one or more lead alloys, the method comprising a step of adding to said bath an additive in the form of a flux-cored wire composed of a metal sheath and a finely-divided filling material, the latter being formed of a metallic lead powder and/or lead alloy powder and of a powder of a material able to release a gas, which is inert with respect to the liquid steel, at the temperature of the bath of liquid steel, said metallic lead powder and/or lead alloy powder consisting of a particle size fraction G_(R) between 200 μm and 500 μm and having the following characteristics: through a 200 μm sieve: G_(R)≦5%; through a 300 μm sieve: 90%≧G_(R)≧10%; through a 400 μm sieve: 40%≦G_(R)≦100%; through a 500 μm sieve: 100%≧G_(R)≧90%.
 10. Method according to claim 9, wherein 0.1 to 10 kg of flux-cored wire is added per tonne of liquid steel to be treated.
 11. Method according to claim 10, wherein the flux-cored wire is added to the bath of liquid steel at a speed of 50 to 200 m/min, preferably of 100 to 150 m/min.
 12. (canceled)
 13. Rolled steel product having a high lead content and containing lead nodules smaller than 100 μm, obtained by the method according to claim 1, characterised in that, when the distribution of the nodules is defined corresponding to the following formula: $I_{R} = {\left( {100/D} \right) \star \left( {\sum\limits_{i = 1}^{NI}{\left( d_{i} \right)/{NI}}} \right)}$ where: I_(R): distribution index D: diagonal of the analysis zone D_(i): minimum distance between the closest lead nodules NI: number of lead nodules with an associated minimum distance, I_(R) is greater than 1.4%.
 14. Additive according to claim 2, wherein the metal sheath is from 0.1 to 1 mm thick, preferably from 0.2 to 0.5 mm thick. 